<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        .a {
            width: 200px;
            height: 200px;
            background-color: orange;
            transition: .3s;
        }

        .a:hover {
            /* 数字  大过于高度的一半  就是圆或者 半圆*/
            border-radius: 100px;

            /* 百分比  大过于50%  椭圆*/
            border-radius: 90%;

            /* border-radius: 10px 20px 30px 40px; */

            border-radius: 100px 100px 100px 0;
        }


        .b {
            width: 200px;
            height: 100px;
            background-color: #ff0;
            border-bottom: 5px solid #000;
            border-radius: 0 0 100px 100px;
        }
    </style>
</head>

<body>
    <div class="a"></div>

    <div class="b"></div>
</body>

</html>